Significance Although viruses which are a combination of SIV and HIV (SHIV) will be very useful for testing HIV vaccines, most do not cause disease in monkeys and thus may not be a realistic test of vaccine potency. Objectives The present study was designed to determine if a SHIV (SHIV89.6 PD) that has been passaged through several monkeys was pathogenic and if it could be transmitted to monkeys by vaginal inoculation. This route of transmission closely models the most common route of HIV transmission, heterosexual contact. Results Two rhesus macaques were inoculated intravenouslly with 25,000 50% tissue culture infectious doses, both became infected and died of AIDS. Seven rhesus macaques were inoculated intravaginally with 25,000 50% tissue culture infectious doses. Six of the 7 animals became infected and had high levels of viral antigen in plasma at 2 week s post-inoculation (PI). In addition, the 6 infected animals had dramatic declines in numbers of CD4+ T cells in lymph nodes and blood by 3 weeks PI. By 14 months PI, 4 of the 6 infected animals have died of AIDS with multiple opportunistic infections. One of 2 rhesus macaques inoculated intravaginally with 2,500 50% tissue culture infectious doses of SHIV 89.6 PD became infected and died of AIDS. Future Directions We have identified a stock of SHIV 89.6PD which transmits by vaginal inoculation and causes disease in rhesus macaques. This virus can be used to vaginally challenge animals immunized with HIV- envelope vaccines. KEYWORDS SHIV virulence, viral pathogenesis, SHIV vaginal transmission